Short answer: Basal body temperature rises by approximately 0.3–0.5 °C after ovulation, but the increase is only visible after the egg has already been released. The chart is therefore best suited for identifying patterns over several cycles and confirming that ovulation has occurred, not for predicting the day in advance. The method works least effectively with irregular cycles or fluctuating work hours.
How do you measure basal body temperature?
Basal body temperature is the body's lowest resting temperature. You measure it immediately upon waking, before getting up or doing anything else. A few rules make the chart easier to read:
- Use the same thermometer every day. A basal thermometer displays tenths of a degree and is more accurate than a standard digital one.
- Choose a measurement site (in the mouth, vagina, or rectum) and stick with it for the entire cycle.
- Measure at approximately the same time every morning, while relaxed and still.
- Start on the first day of your period and record each value on a chart, on paper or in an app.
- Note anything that may affect the reading: fever, short or poor sleep, stress, alcohol, or certain medications.
What happens to the temperature around ovulation?
Before ovulation, the temperature is usually around 35.5–36.7 °C. Just before the egg is released, it may drop slightly, and approximately one day after ovulation, it rises. The increase is due to progesterone from the corpus luteum, which forms where the egg was released, and it remains elevated until your next period approaches. You can read about the length of this phase in the article on the luteal phase and progesterone.
| Part of the cycle | What the chart shows |
|---|---|
| Before ovulation | Lower level, often 35.5–36.7 °C, with a possible slight dip just before |
| Approximately one day after ovulation | Rise of approximately 0.3–0.5 °C |
| Until your next period | Elevated level that remains |
Why does the chart come too late to time intercourse?
The egg can be fertilized approximately one day after it is released, while sperm can live in the body for up to approximately five days, according to 1177. The most fertile days therefore occur before ovulation, and the temperature chart does not reach that point.
This is shown in the classic study by Wilcox and colleagues in the New England Journal of Medicine. Among 625 cycles with a known day of ovulation, pregnancy occurred only after intercourse during the six days ending on the day of ovulation. The probability of fertilization was approximately 0.10 after intercourse five days before ovulation and approximately 0.33 on the day of ovulation itself. A later analysis by Dunson and colleagues, which included couples in whom ovulation was identified using basal body temperature, found the same six-day window. The highest probability was on the day before ovulation and fell to nearly zero after it.
When the temperature has risen, the best days have therefore usually already passed. If you want to find the window in advance, the length of your cycle, changes in cervical mucus, and LH tests are more useful clues, which we cover in the article on finding your fertile period. Intercourse two to three times a week is the general recommendation from 1177 when trying to conceive.
Who can benefit from a temperature chart?
After a few cycles, the chart shows whether you appear to be ovulating and approximately how long after your period it occurs. It requires patience: several cycles are needed, and the method works best with a regular cycle. Those who work irregular hours or have irregular periods often get a chart that is difficult to interpret. Because the method can be cumbersome and imprecise, it is usually combined with cervical mucus observation or ovulation tests.
If the temperature remains elevated for 18 days or more after the estimated day of ovulation, it may be due to pregnancy, but this is determined with a pregnancy test. When the test results can be relied upon is described in the article on pregnancy tests. Basal body temperature does not work as reliable contraception.
Frequently Asked Questions
How many cycles do I need to keep a chart for? Plan for several months. A single cycle tells you little, as fever, sleep, and alcohol can affect individual readings, and the pattern only becomes clear when several cycles are compared.
Can I measure with a regular thermometer? Yes, a standard digital thermometer can be used. A basal thermometer displays tenths of a degree, making the slight rise easier to see.
Does it matter where I measure? The important thing is to measure in the same place and with the same thermometer throughout the cycle, so that the readings can be compared.
Read also
- Find your fertile period – cycle, body signs, and tests
- The luteal phase and progesterone – length, testing, and treatment
- How long does it take to get pregnant – and when should you seek help?
General information, not medical advice. Consult healthcare professionals about your own situation.
Sources
- Invitra: What is the basal body temperature method and how is it performed?
- HealthLink BC: Basal body temperature
- 1177: Fertilization
- Wilcox, Weinberg, Baird. Timing of Sexual Intercourse in Relation to Ovulation. N Engl J Med 1995;333:1517–1521
- Dunson et al. Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation. Hum Reprod 1999;14:1835–1839